Two major problems encountered in long-haul fiber-optic communication systems are pulse spreading and attenuation. Pulse spreading is caused by chromatic and modal dispersion, and attenuation is caused by losses in the transmission medium. Modal dispersion can be eliminated by the use of single-mode fibers. Chromatic dispersion can be reduced by using a narrow-band source, a low-dispersion fiber, a dispersion-shifted fiber in combination with a source close to the zero dispersion wavelength, or a dispersion transformer. Attenuation can be reduced by careful selection of operating wavelengths and fibers. However, even with state-of-the art selections, there are limits to the distance over which a fiber-optic cable can carry a signal, and on the bandwidth-distance product of fiber-optic data links.
One method to increase the distance over which a fiber-optic data link can operate, as well as increasing the distance-bandwidth product, is to use repeaters along the communication link. A repeater is a device that receives the optical signal, amplifies it, and then retransmits it. In addition, a repeater may also perform pulse shaping. Currently used repeaters for fiber-optic communications systems operate by converting received optical input pulses into corresponding electrical signals, then amplifying and shaping the pulses in the electric domain, and finally converting the modified pulses back into an optical signal for retransmission. Pulse amplification is most important when optical losses are the dominant problem with the data link. If dispersion is a problem, reshaping of the pulses can result in significant improvements in reliable data transmission.
There are two principal difficulties with conventional repeaters that rely on conversion of the optical pulses to the electrical domain. First, the relatively slow speed of electrical components limits the speed at which the repeater can operate. Second, the use of electrical components tends to increase the susceptibility of the repeater to radiation induced noise. Susceptibility to radiation, such as ionizing radiation, can be minimized only by relatively cumbersome and expensive designs. There is therefore a need for a repeater for fiber-optic communication systems that avoids the use of electronic components in the signal path.